The invention relates to an electric discharge tube comprising in an evacuated envelope a grid electrode having a substantially flat grid of pyrolytic graphite which is provided in a grid holder.
An electric discharge tube is a tube in which a beam or a flow of electrons and/or ions is generated, for example, an ion source, a cathode-ray tube, a travelling wavetube or a transmitter tube. Pyrolytic graphite is a synthetic form of carbon which is obtained on a suitable substrate or mandril by deposition of elementary carbon from a carbon-containing gaseous phase. By previously determining defined deposition parameters it is possible to manufacture layers of pyrolytic graphite which are distinguished by a pronounced anisotropy of a number of physical properties. A detailed description of the deposition process is found, for example, in "Carbon" 5 (1967), pp. 205-217 and in "Philips Technisch Tijdschrift" 28 (1967), pp. 133-144.
A method of manufacturing a grid electrode having a flat pyrolytic graphite grid is disclosed in U.S. Pat. No. 4,245,379. This patent describes how a flat pyrolytic graphite grid can be obtained by cutting a disc from a cylinder of pyrolytic graphite, then grinding it, providing it with apertures and stretching it in a grid holder. Such a method was necessary because previously it had proved impossible to manufacture directly thin flat pyrolytic graphite grids having a thickness of less than 100 .mu.m by means of epitaxial growth on a hot mandril. This was impossible as a result of stresses which were generated in the grown layer during the cooling process. The method described in patent specification No. 4,245,379, however, has the disadvantage of being very laborious and the grid has to be stretched in a grid holder. Another disadvantage is that the heat transfer from the grid to the holder is not optimal so that at high thermal loads the possibility exists of the grid becoming too hot.